Sodium transport and gluconeogenesis in the kidney are stimulated by mineralocorticoids (e.g., aldosterone) and glucocortoids (e.g., corticosterone), respectively, and inhibited by synthetic anti-mineralocorticoids (spirolactones) and progesterone. Responses to physiological concentrations of the hormone agonists and antagonists are mediated by specific cytoplasmic receptor proteins. Receptor-agonist complexes undergo "activation", migrate into the nucleus, bind to chromatin and modulate transcription. Precise physical-chemical characterization of the receptors (molecular size, symmetry, subunit composition) and identification of the separate functional domains (steroid-binding, chromatin-binding) will contribute to a deeper understanding of their physiological roles. We will separate the minerolocorticoid from glucocorticoid recetors of kidneys of adrenalectomized rats, guinea pigs, and small pigs, and purify several forms of each: cytoplasmic receptors plus or minus activation, nuclear receptors and their specific chemical cleavage products. Techniques will include interconversion of receptor forms, covalent cross-linking of subunits, systematic electrophoresis, electrofocusing, ion exchange and DNA-cellulose chromatography. Affinity-labeling of the receptors with 3H-steroid derivatives will permit purification under denaturing conditions and identification of the peptides near the steroid-binding site. Covalent labeling of all the proteins with another isotope (e.g., 14C-methylation of lysines) will facilitate protein assays during the 10,000- to 100,000-fold purification of receptors. The molecular basis for dissimilar responses to hormones that compete for the same binding sites will be explored by comparing the observed saturation and confronations of the receptors in the presence of agonists, antagonists and partial agonists with the predictions of my mathematical model for combined steric and allosteric interactions and alternative models. The widespread use of corticosteriods in treatment of benign and malignant disease and their profound effects on tissues othe than the kidney (blood cells, bone, skeletal and cardiac muscle, skin, liver mammary gland) amplify the significance of the proposed studies.